A high speed turbo machine, such as, for example, a steam or gas turbine, generally comprises a plurality of blades arranged in axially oriented rows, the rows of blades being rotated in response to the force of a high pressure fluid flowing axially through the machine. Due to their complex design, natural resonant mechanical frequencies of the blades may coincide with or be excited by certain blade rotational speeds and rotational harmonics thereof. To prevent excessive vibration of the blade about its normal position, prudent design practice dictates that the blades be constructed such that the frequencies of the lowest modes fall between harmonics of the operating frequency of the turbine. In addition, the blades may be excited by non-synchronous forces such as aerodynamic buffeting or flutter. In order to avoid the vibration exceeding certain levels and setting up objectionable stresses in the blades, it is common to monitor the vibrations of the blades, both during the design and testing of the turbine and during normal operation of the turbine. For example, it is known to use non-contacting proximity sensors or probes to detect blade vibrations. The probes detect the actual time-of-arrival of each blade as it passes each probe and provide corresponding signals to a blade vibration monitor system (BVM). Small deviations due to vibration are extracted, from which the BVM may determine the amplitude, frequency, and phase of the vibration of each blade.
The measured vibration amplitude is highly dependent on correct positioning of the sensor above the blade target, which may comprise a target affixed to the blade or a feature of the blade. Although every effort is made to ensure that the installation of the sensors on the cold casing of the turbine locates the sensors over the targets in the hot running state, it is still necessary to install redundant sets of sensors located at different axial positions to ensure that at least one set is correctly positioned and usable when the turbine is operating. For example, in a known construction for steam turbine blades, the target strip at the end of the blades may be approximately 1.5 inches long, but the target is oriented at a small cant angle relative to the plane of the blade row such that the target extends only about 0.5 inches in the axial direction. As the turbine heats to its operating temperature, the blades and associated targets shift axially with thermal expansion of the rotor and casing, and it is necessary to accurately predict and locate the sensor over the relatively narrow axial extent of the target strip at the heated operating temperature of the turbine.
The installation of additional sensors has proven to be costly and time consuming. In addition, it has been found that it is not necessarily possible to determine the validity of a sensor position by examination of the signal produced by the sensor, unless the positioning of the sensor is extremely poor, such as off the target. A poorly positioned sensor producing an apparently valid signal can result in invalid or poor data being provided to the BVM and an incorrect analysis of the blade vibration reading.
Accordingly, there is a need for a method of determining that a sensor is correctly positioned to provide valid sensor signals in response to target passing events.